Paper and paper board are manufactured of a fiber suspension, so called paper making stock or furnish, by draining water from the fibrous suspension through a wire or wires of a paper or board making machine. The stock may comprise a number of fibrous components, like for instance chemical pulp, chemimechanical pulp, mechanical pulp, and recycled pulp, and various additives, like fillers, retention aids, sizing agents, paper dyes, wet-strength and dry-strength enhancing chemicals, just to name a few.
Fillers are usually finely divided mineral products, normally in the size range of about 0.5 to 5 micrometers. The main function of the fillers is to reduce materials cost per unit mass of paper as the fillers are considerably cheaper than papermaking fibres, other functions are to increase opacity and to increase smoothness of the end product. The type of paper or board to be produced has a huge effect on the choice of filler, blends of fillers, and their level in the product. The most important fillers are calcium carbonate and kaolin clay aka calcined kaolin. Most fillers are delivered to paper mills either as dry powders or as slurries. One form of calcium carbonate, i.e. Precipitated Calcium Carbonate (PCC) is often prepared at a plant adjacent to the paper mill, on-site, and delivered to the paper machine as a slurry. Lately, an in-line production of PCC has been successfully tested in mill-scale applications. By the in-line production of PCC is meant a process in which the carbonation of the milk of lime (MOL) is performed by means of pure carbon dioxide or carbon dioxide containing gas in the presence of the stock in the pipeline taking the stock or furnish to the headbox of the paper or board machine.
The milk of lime may be produced by calcining limestone (calcium carbonate) at high temperature to drive off CO2, and slaking the resulting lime (calcium oxide) by addition of water to form a lime suspension (calcium hydroxide).
A downside in the use of fillers relates to the fact that while, as mentioned already above, the fillers increase the opacity, which is a desired feature of paper or board, the fillers also reduce the strength of paper or board. Thus, the paper manufacturer has to find a balance between opacity and strength. One way to maintain good product strength with good optical properties is to use high quality fillers. A good example of high quality fillers is calcined kaolin, which is an anhydrous aluminium silicate produced by heating ultra fine natural kaolin to high temperatures in a kiln. In the calcinations process, the water of hydroxylation is first driven off as vapour at temperatures of 500-700° C. Then heating is continued up to 1000° C. where ultra fine particles begin to agglomerate into larger particles. The final result is kaolin—air interfaces with a relatively large internal pore volume. After the calcination, clay is pulverized to remove any oversized agglomerates. The end product typically has a very narrow particle size distribution. Calcined kaolin is defined as specialty filler and is mainly applied in order to increase light scattering and opacity and also to reduce potential ink print through. However, the average price level of calcined kaolin is about 3-fold compared to PCC. In practice this means that the paper manufacturer could use three times more PCC than calcined kaolin. However, such a high increase in the use of PCC would inevitably mean drastic reduction in paper strength.
A further downside in the use of fillers is that they tend to interfere with inter-fiber bonding, reducing the strength of paper. Another downside is the tendency of small-sized fillers to pass the paper machine wire and end up in the filtrate. Filler retention is a term describing the proportion of the fillers remaining in the web on the wire. The lower the value is the weaker is the retention. A further downside is the tendency of the fillers to form agglomerates decreasing the quality of the paper. Thus there are a few traditional rules for the use of fillers, i.e. (a) making sure that the filler material is fully dispersed into individual particles before it is added into the paper making stock or furnish. Sometimes a specific chemical, i.e. dispersant, is used for the above purpose. (b) mixing it with the furnish at a location that does not adversely affect other additives, and (c) retaining it in the fiber mat on the paper machine wire. The first and the last goal may sometimes be in conflict with each other, especially if a large amount of dispersant has been used to create a stable suspension. Also, it is obvious that the finer material the filler is the more easily it will be filtered out of the web and wire, and the weaker is the retention. To improve the retention the furnish is provided with retention agent/s for flocking the filler to the fibres and other solids in the furnish. From the above it should be understood that further chemicals, like dispersants, strength enhancing chemicals and retention aids are in everyday use for facilitating the increased use of fillers.
Further, chemicals enhancing the strength of paper, i.e. internal bond, burst strength, tensile strength etc. are also used and they include various natural and man-made or synthetic polymers. One of the most widely used strength enhancing chemical is starch. The starch may be based on any raw material, e.g. potato, maize, wheat, tapioca, rice, corn, waxy maize or waxy corn. Carboxymethyl cellulose (CMC) and guar gum derivatives are the most popular natural polymers.
PAAE (Polyamideoamine-epichlorohydrin), c-PAM (cationic polyacrylamine polymer), a-PAM (anionic PAM), silicate, nanoparticles, copolymer of polyvinylamine and polyacrylate (PA), anionic copolymers of acrylamide or other acrylamide polymer are widely used examples of the synthetic strength enhancing chemicals.
The distinction between retention chemicals and strength enhancing chemicals is, in practice, negligible, as the working principle of both chemicals is the same, and they are introduced into the furnish at about the same time, i.e. to the furnish upstream of the headbox of the paper machine
WO-A1-2007/067146 discusses a method of producing on-site precipitated calcium carbonate (PCC) for use as a filler in paper or paper board production, wherein the carbonation of calcium hydroxide is performed in the presence of starch. The precipitated calcium carbonate produced by the above described method and when used as filler decreases the dusting tendency of the paper and increases the strength of the paper or paper board. It is believed that the above results are due to the starch binding the small-scale filler particles such that they are not loose in the paper, and cannot, thus, cause dusting. In the process discussed in the WO document the PCC is produced on-site as the presence of fibres and fines are believed to disturb the incorporation of starch in the PCC particles. The PCC-starch mixture is transported for use at the paper mill by means of pumping or by means of a tank truck.
U.S. Pat. No. 2,188,494, U.S. Pat. No. 3,443,890 and U.S. Pat. No. B1-6,294,143 discuss basically similar methods of manufacturing PCC in the presence of starch or, in broader sense, in the presence of carbohydrates.
Yet, when considering the production of PCC in the presence of starch, or carbohydrates, discussed in prior art publications, for instance the above mentioned WO document teaches that the precipitated calcium carbonate is most preferably not produced in the presence of fibers, since fibers and fines can disturb the incorporation of the starch and/or the carboxy methyl cellulose in the PCC particles. Moreover, the production of precipitated calcium carbonate separated from the paper-making process, i.e. not in-situ during paper-making, makes it easier to control the process. In other words, in prior art there is a clear prejudice that starch loses its ability to bond to PCC particulates when fibers or fines are present, whereby it could be expected that, in order to ensure sufficient bonding to fibres, i.e. for improving the retention of PCC, and strength of paper or board, an overdose of retention/strength enhancing chemical should be used.
WO-A2-2009103853 discusses introduction of thick stock components into the furnish by means of an injection mixer. The document also suggests that while injecting thick stock also chemicals, like milk of lime could be introduced together with a thick stock component. The document further teaches the introduction of milk of lime and carbon dioxide into the furnish prior to the headbox screen and the retention chemical together with fine fraction of the fiber recovery filter after the headbox screen such that the precipitation of PCC has taken place prior to the injection of the retention chemical. The document discusses further how various additives may be mixed beforehand with a thick stock component i.e. prior to injecting the thick stock component in the furnish.
WO-A2-2009103854 discusses precipitation of PCC in the short circulation of a fiber web machine. The document suggests introducing all or at least substantially all chemicals needed in the manufacture of a fiber web after the precipitation of PCC into the furnish.
DE-A1-102007029688 discusses methods of producing PCC for the paper manufacture. The document teaches the preparation of PCC in a separate reactor in which water, milk of lime, carbon dioxide and nuclei for crystallization are introduced. The document teaches that as the nuclei, fines, fine impurities, retention agent, starch, etc. may be used. After the PCC is precipitated the suspension is introduced to be mixed with the fibrous constituents of the furnish, i.e. paper making fibers, for instance. Thus the document teaches in the manner of earlier discussed WO-A1-2007/067146, the precipitation of PCC in the presence of, among other options, starch or retention chemical. However, the discussed method has a few drawbacks. Firstly, allowing the PCC crystals to precipitate on the fines, fine impurities etc. lead to small-sized particles that are relatively inactive and apt to being filtered in the white water in the paper manufacture unless being bonded to the fibers by retention chemicals added later on in the furnish. And secondly, if the nuclei are comprised of retention chemical molecules, the molecules will be, during the precipitation reaction, surrounded by the PCC crystals such that the molecules have no or at least very little free surface for attaching to the fibers when, later on, getting into contact with such.
Thus the main problems with the prior art are                High investment, energy, running and maintenance costs involved in the PCC production in an on-site facility,        Increased use of high quality expensive fillers if both high opacity and strength are required,        Increased use of retention/strength enhancing chemical if both high opacity and strength are required, and        Uneven homogeneity of the PCC crystals.        